Today, magnetic resonance imaging (or "MRI") is a widely used imaging technique for looking inside the human body without the need for X-rays or invasive surgery. Magnetic resonance imaging uses the phenomenon of nuclear magnetic resonance (or "NMR") in which radio signals generated from atomic nuclei subjected to a strong magnetic field are detected and used in forming images of bodily portions of the subject.
Nuclear magnetic resonance takes advantage of the fact that certain atomic nuclei having an odd number of protons and/or neutrons (such as hydrogen nuclei) have magnetic moments or "spins" which, in the presence of a strong magnetic field, tend to align themselves in the direction of the magnetic field. When the nuclei are exposed to radio waves with a certain frequency, known as the resonant or Larmor frequency, the vectors of the nuclei become displaced out of alignment with the magnetic field. After such displacement, the spins of the processing nuclei produce infinitesimal radio signals or magnetic resonance signals, also at the resonant or Larmor frequency. For example, when hydrogen nuclei are placed within a magnetic field of 10 kilogauss (1 tesla), they can be made to resonate at a Larmor frequency of 42.576 MHz. The spinning nuclei eventually drift back into alignment with the magnetic field and the magnetic resonance signal gradually decays.
With MRI, the properties of the acquired magnetic resonance signals from individual small volume elements or "voxels" within a larger subject are determined in order to generate a graphical or pictorial image representing individual voxels having different chemical and/or physical properties which can be displayed in contrasting shades or colors. Because the magnetic field and applied exciting radio frequency signal are usually applied to a large region of the subject, however, it is necessary to use further techniques for spatial encoding purposes, i.e., for identifying the NMR signals acquired from different volume elements to generate the MRI image for the larger region under study. Such techniques typically include the further application of gradient magnetic fields to make use of the property that nuclei in a stronger magnetic field will resonate at a higher frequency than nuclei in a weaker magnetic field. Using computer analysis, the acquired signals received in time (i.e., time-domain signals) can be converted into frequency-domain signals using Fourier transformation. The frequency-domain signals indicate signal strength as a function of frequency and therefore contain information about signal strength as a function of position. In other words, a signal at a given frequency must have come from nuclei in the region of the magnetic field with the corresponding field strength. Such spatial encoding techniques are well known to those skilled in the field of MRI and are beyond the scope of this disclosure.
The acquisition of MRI images is a complex process and requires high precision apparatus, which presents many difficulties and challenges to designers of MRI equipment. One difficulty that MRI designers face is maintaining the stability of the applied static magnetic field. Not only must the static magnetic field be strong for MRI acquisition, typically in the range of from 1 kilogauss to 15 kilogauss (1.5 tesla), but the field must also be precisely configured and uniform so as to be constant to preferably at least one part per million to provide acceptable images.
A major problem with keeping the static magnetic field constant occurs from the drifting of the magnetic field or disruptions to the field caused by external conditions and forces, such as temperature variations, power supply stability and aging of the components, in addition to the movement of ferromagnetic trains or subways, surges in power lines, elevators and underground garages. These and other similar disruptions can move or pull the static field away from its desired resonance value. This problem is especially prevalent in MRI systems which are employed in urban areas which also contain many such external factors (e.g., subways, trains, etc.) that can affect the static magnetic field.
Changes to the static magnetic field during the imaging process are problematic and can cause "ghosting," blurring and other artifacts in the acquired images or even totally destroy the imaging data acquired during an imaging session. With scan times typically lasting as long as 2 to 10 minutes, deviations in the static field which alter the acquired MRI data can not only cause great inconvenience to the patient and imaging personnel, but also results in additional expense (ultimately borne by the patient or healthcare provider) if the imaging procedure must be repeated. Accordingly, there is a great need to stabilize the static magnetic field in a MRI system.
Magnetic field stabilization methods have been used in the field of NMR spectroscopy for a number of years. One such method of stabilizing a magnetic field for an NMR spectrometer is disclosed in U.S. Pat. No. 4,110,681 to Hofer et al., which provides a lock system for maintaining and stabilizing the magnetic field at a constant strength. This patent discloses periodically pulsing a control sample to produce a series of free induction decay (FID) signals, the periods of which are determined and used to correct the magnetic field. Each FID signal is analyzed by measuring the time it takes for the signal to cross the zero axis a given number of times. This measured time is then compared to a predetermined time, and if the times differ, the current flowing through the field coils is varied in the direction tending to eliminate the difference. A significant problem, however, with the stabilization method of Hofer et al. is that there is a sacrifice in accuracy since the system only accounts for integer values of zero crossings in the FID signal. In other words, the correction technique loses accuracy when the sensed FID signal is not exactly in phase with the desired FID signal such that the magnetic field has drifted, but there are still the same number of zero crossings in the sensed FID signal. For example, a shift in the static magnetic field causing an 89 degree phase change in the sensed FID signal may not generate an additional zero crossing and hence no correction signal will be applied even though the static magnetic field has drifted.
In Hoult et al., "A Novel Field-Frequency Lock for a Superconducting Spectrometer," 30 J. Mag. Res., 351-65 (1978), ("Hoult"), the background of providing NMR spectrometers with some type of feedback system to account for variations of the magnetic field is described. Hoult describes prior systems that use the output voltage of a continuous-wave spectrometer when very close to resonance, which is proportional to the error in the resonance condition, as a feedback mechanism. Hoult discusses that a superconducting solenoid is less immune to external influences than traditional NMR spectrometer systems using permanent magnets.
Static magnetic field stabilization based on such NMR spectroscopy "lock" techniques have now been introduced in the field of MRI. Magnetic field stabilization based on NMR techniques uses the NMR signal from a separate fiducial or from a slice outside of the imaging volume. The usual imaging sequence is modified to first allow time for RF excitation of the sample (or slice) to obtain the NMR signal. The NMR signal following the RF excitation pulse is amplified and demodulated to obtain an audio or low frequency signal. The low frequency signal is integrated with an analog integrator and the integrated value is used as an input voltage to a programmable power supply to drive a set of correction coils placed within the imaging volume of the magnet to stabilize the magnetic field. When the magnet is locked, the above-described system pulls the magnetic field slightly away from the resonant frequency and the area of the of the low frequency signal (or nearly-zero beat signal) is zero. Whenever the magnet drifts from resonance, the integrated value becomes non-zero and the output of the integrator acts as a correction signal to the correction coils to pull the magnet back to its intended resonance value.
The implementation of NMR lock techniques to the field of MRJ, however, has been demonstrated to be problematic and impractical in clinical settings due to a number of problems. For instance, problems occur due to the use of the analog integrator which can introduce errors into the correction signal from the baseline drift of the integrator. These prior NMR lock techniques also lack sophistication in handling data in a number of situations, such as the inability to discard data in the event of a large magnetic field disturbance or drift, and the inability to even detect large magnitude magnetic field drifts. The prior techniques further experience problems when trying to pull the magnetic field back to the original resonance value after a large magnetic field disturbance.
Therefore, there exists the need for an improved method for stabilizing the static magnetic field in an MRI system which overcomes the problems associated with the aforementioned prior art methods and systems.